Comparative advantage would be correct. I'd say first of all were the geographic and climatic factors. New England had many fine natural harbors, and easy access to hardwoods needed for shipbuilding. The second was the relatively poor rocky soil and long winters of New England vs. the rich soil and longer growing season of the south.
In New England, capital tended to flow to shipping and mercantile activities while in the South capital tended to flow to large agricultural ventures.
Those factors did lead to different attitudes with different economic incentives.
New Englanders might have had to turn to the sea for a living in some cases .... not that their crops were dying in the fields as it was. New Englanders grew up on home-grown lentils (a better diet, btw, it's been pointed out in an American history course, than that of Southerners who lived on cornbread, ham, and bacon -- and soft water, which denied them an important source of minerals -- leading to shorter life expectancies).
But the South didn't lack for world-class estuarine anchorages (Norfolk, Charleston, Savannah, Jacksonville -- not during the colonial period, of course -- and the Carolina sounds), and timber interests logged enormous trees out of the areas around Great Smoky for over a century -- I'm talking about hickory logs eight and nine feet in diameter. (There's a house at Great Smoky National Park built of logs that big. Some "log cabin"!)
Rhett specifically charges undercutting, but he doesn't give details. I've seen another early-20th-century source that accused New England of capturing the cotton trade after the Civil War. I have a copy of it "somewhere" on media. If both charges are true, that would be enough to establish a "pattern and practice" of commercial predation by Northern mercantile interests. By what mechanisms, I'd like to know, if it's true at all and not just a bunch of crying by runners-up.